The present invention relates to the field of radio transmission, and more particularly to the field of military tactical transmission. Transmission of such a type needs to be able to satisfy constraints in addition to those of conventional radio transmission; it must be capable of being implemented on numerous types of transmission channel, and in particular on frequency-selective channels. It must be discreet, and it must withstand jamming as well as possible. Finally, it is necessary for transmission of this type to guarantee that the system is robust in terms of ease of acquiring and maintaining synchronization.
To withstand jamming, it is known to use spectrum-spreading techniques. These techniques are described, for example, by M. K. Simon et al. in xe2x80x9cSpread spectrum communicationsxe2x80x9d published by Computer Sciences Press, 1988. One of the problems with such spreading techniques is that when the signals are unspread, a fraction of the energy from the jammer is recovered in the form of noise. This can be a problem with very high power jammers.
Another problem with spreading techniques is the usual assumption that the transmission channel is without distortion no longer applies to the broad band of a spread signal. Solutions have already been proposed to this problem, and the receiver known as a xe2x80x9crakexe2x80x9d receiver makes it possible in particular to eliminate disturbances due to multiple paths in the spread signal. This technique is described, for example, by John P. Proakis in xe2x80x9cDigital communicationsxe2x80x9d, third edition, published by McGraw-Hill International Editions, Electrical Engineering Series, 1995.
FIG. 1 is a block diagram of a known spreading technique. The symbols Si of the signal of period T are initially oversampled, prior to be multiplied by an encoding sequence, e.g. a sequence PN of pseudo-random noise. Chips ci,j of period T/N are then transmitted over the channel. On reception, synchronization is performed using the self-correlation properties of the sequence PN, after which it is possible to multiply the received chips ĉi,j by the sequence PN to obtain the estimated symbols Ŝi.
A signal occupying a frequency band 2B is spread out into a signal occupying a frequency band 2NB. FIG. 2 shows the appearance of the initial spectrum of the symbols in dashed lines, and the appearance of the spectrum after spreading in continuous lines.
For modems having a plurality of spreading modes, of the kind used in military tactical transmission systems, it is conventional to propose two different types of equalization algorithm. A first type of algorithm is used for non-spread signals, while another type of algorithm, e.g. the algorithm implemented in a xe2x80x9crakexe2x80x9d receiver, is used for spread signals.
It is also known to perform multi-carrier transmission using spectrum spreading, as shown in FIG. 3. As is in FIG. 1, the symbols Si are oversampled, and then multiplied by the sequence PN; however, the chips are not sent directly over the channel as in the case of FIG. 1, but they are demultiplexed and distributed over a plurality of subcarriers; the chips dik for transmission on subcarrier k are modulated thereon by being multiplied by exp (2jxcfx80fkt); the modulated subcarriers are then summed prior to being sent over the channel. On reception, it is necessary to perform synchronization in time and in frequency on the various subcarriers so as to enable the various chips to be reconstructed in phase. Thereafter, the chips can be recovered on each subcarrier and then remultiplexed, after which the procedure is the same as for reception in the diagram of FIG. 1. An example of such a technique is given by A. Chouly et al. in xe2x80x9cOrthogonal multicarrier techniques applied to direct sequence spread spectrum CDMA-systemsxe2x80x9d, Globcom 1993, pp. 1723-1728.
In civilian applications, where discretion is not a problem, synchronization solutions are known. For time synchronization, it is possible to stop the signal from time to time, and then to perform envelope detection at the receiver. For frequency synchronization at the receiver, it is possible to send a subcarrier with greater power at certain times, or to extinguish all of the subcarriers except for one at a given moment. Those solutions have the advantage of being easy to implement. However, they do not satisfy the discretion constraint that applies to military systems. In addition, they are not appropriate to getting rid of any jamming.
When the technique of multicarrier modulation with cyclical extension is used, it is also possible in some cases to use known algorithms that make it possible to recover the frequency and the clock rate from the subcarriers. In addition to their specific character, those solutions do not operate at low signal-to-noise ratios, which are typical of spectrum spreading.
In conventional manner, the functions of demultiplexing, of modulating the subcarriers, and of adding together the modulated subcarriers can be performed by the fast Fourier transform, as represented by the curly brace on FIG. 3. As a result, cardinal sine (sinc) subcarriers are obtained of width 2N/MT and spaced apart by N/MT, using the same notation.
In dashed lines, FIG. 4 shows the spectrum of the spread multicarrier signal obtained in a circuit of the type shown in FIG. 3; frequency is plotted along the abscissa which is graduated in multiples of 2NB/M. The ordinate gives the spectrum power density in dB. A spectrum is obtained that is made up of M arches of unit width 2NB/M, where M is the number of subcarriers. As can be seen in the figure, the arches overlap. An appropriate choice for the value M giving the number of subcarriers makes it possible to transmit each subcarrier under conditions where the assumption of a distortion-free channel is satisfied. This makes it possible to avoid the equalization problems encountered in spreading techniques.
R. Vallet and K. Haj Taieb in xe2x80x9cSpaced multicarrier modulation, special issue on multicarrier communicationxe2x80x9d published by Ecole Nationale supxc3xa9rieure des txc3xa9lxc3xa9communications, 1994, propose a digital implementation of a multicarrier modulation technique using QAM type modulation and a pulse-shaping function with a Nyquist filter. That article proposes associating a fast Fourier transform with a polyphase filter. Such a solution using filtered multiple carriers still implies synchronization which is neither discreet nor robust, as explained above.
The invention proposes a solution to the problem of synchronizing subcarriers in a spread spectrum multicarrier transmission system for military applications. It makes it possible to satisfy the discretion constraints that are imposed in military applications.
With differential demodulation, the invention makes it possible to limit the recombination loss caused by the differential demodulation, while still conserving adequate resistance to jamming.
More precisely, the invention provides a spread-spectrum transmission system with multiple carrier modulation in which the signals for transmission over each of the subcarriers are filtered prior to being transmitted over the subcarrier, thereby making it possible on reception to synchronize the various subcarriers in time, and in which each subcarrier carries a plurality of chips that result from spreading a single symbol.
In an embodiment, the various subcarriers are subjected to coherent modulation, and each subcarrier carries a number of chips that result from spreading a single symbol, which number is sufficient to enable the various subcarriers to be brought into phase alignment by recombining chips coming from a single symbol on each subcarrier.
In another embodiment, the various subcarriers are subjected to differential modulation, and each subcarrier carries a number of chips resulting from spreading a single symbol, which number is sufficient to provide resistance to jamming by limiting recombination losses because of the differential modulation.
Advantageously, said signals are filtered by lowpass filters ensuring that the spectra of the subcarriers remain distinct.
The signals can be filtered by Nyquist root filters.